(I am posting this to spite homey.)
Its pretty dead out here yo. Major bombing is only done in certain parts of Tokyo...you'll barely ever see a piece or a throw up, its always tags or stickers. There are two dudes that get up a lot here, Ekys and Wanto but they mostly just tag. People come from all around the world and leave a tag here or there. Awr Crew, Slick, Neckface, Kaws, Solo ASCREW, Haze(has a shop and sells Belton at like 11 a can and shit)...etc.
The rest of Japan has tags around and the occasional piece but like I said, its dead or was never there.
Yokohama has mad permission walls but I have no idea who you would have to talk to, if anyone to paint there.
I have heard that Okinawa has a lot of graff too, I haven't been out that far yet.